Present day telecommunication wireline termination circuits for a variety of data communication systems, such as T1 systems, that are implemented as redundant receiver architectures, are typically configured in accordance with one of two commonly used approaches. A first of these, diagrammatically illustrated in FIG. 1, is to hard-wire a termination resistor 10 across the tip-ring leads 21, 22 of a system backplane tip-ring pair, and then wire-OR each receiver circuit of a pair of redundant line cards 31 and 32 to the tip-ring leads. While this scheme is generally acceptable as long as at least one line card is inserted, it can lead to catastrophic system failure when no card is inserted in the backplane, since transient suppression circuitry for protecting against lightning strikes and power fault surges is located on the line cards, rather than the backplane. For example, the occurrence of such a transient event when no line card is inserted will blow the termination resistor, requiring replacement of the entire system, rather than an individual card.
The second approach, diagrammatically illustrated in FIG. 2, involves placing a termination resistor 40 and an associated (such as but not limited to a digitally controlled analog switch, or a relay) switch 45 at the front end of the receiver circuitry 50 of each line card, adjacent to the backplane 55, and then selectively digitally activating the switch on whichever line card is used to terminate the circuit. This switch closure thereby places the termination resistor 40 across the tip-ring pair 70T-70R to which the line card receiver circuitry 50 is coupled.
When using this approach, care must be taken not to close more than one switch, since doing so would place two or more termination resistors in parallel across the tip and ring leads and thereby terminate the line in a lower and wrong value of impedance. Although this second mechanism eliminates the blown resistor problem of the first approach (since the T1 link is no longer terminated if all cards are removed), it can corrupt data during card insertion and removal, and it adds to the cost of a respective line card by requiring that each line card contain a termination resistor and a digitally controlled analog switch.